Arte is a Japanese anime and, is derived from the manga. Arte is a name of a girl from Florence. Arte was a passionate girl who followed her dreams and was a feminine figure back in the renaissance era.
The start of Arte's feelings for Leo
This anime is much more focused on a feminist point of view and romance is not the focus of the story. Even though it's true, we still can see the feelings Arte was growing for her master Leo in many scenes.
Leo owns an atelier and is also known as a scary or devil person by looking at his personality and the way of his talks. He seems to be the cruel type, but, later we see the warm side of his personality. Leo is the only one who never did mention to Arte, that she is a woman and so the workload would be less and the behavior must be less horrible.

Arte, when she got accepted as Leo's apprentice, she was excited and determined to start her journey. Leo saw himself when Arte was telling her the reason why she wanted to be a painter. The reason was similar as to why they wanted to start their career in the field of arts.
Scenes when Arte realized her feelings

During the period of Arte's learning as an apprentice, Leo asked him to change into the clothes of a man and to go out with him as a task. It was a carnival season going on at that time and while they were out, Arte lost him for a while and when she found him, he was there with a woman and it was the first time for her to see him smiling and she was there standing gazing at him.
Leo took her to a place where a man was performing an autopsy and, Leo told her that to sketch a human body, one should be aware of the structure of the parts. Later when that man took out a heart from the body, a man faints and, Leo wondered if Arte was doing alright and he was going to ask her, he saw the happiness and determination on her face as she was doing her work quite alright and that made Leo think of her undeniable guts.
While Arte was focusing on her work her hat fell and everyone recognizes that she was a girl and everyone started frowning at her. Leo snatched her and ran away while putting her on his shoulder which made Arte a little uncomfortable and shy. When the people came near them they went into some small alley and hid.
Leo put Arte between his legs and her face into his chest while holding her by both his arms. Arte saw a man this close for the first time and was amazed to see his line and his handsome face of his. This was the moment when she first felt her heartbeat beat so fast as if it was going to pound out from her chest.
Another day and another task await her as time passes. Leo asked her to redo her room which was all shattered with broken woods and doors. Arte had to do all that alone and she did a great job. Leo patted her head and complimented her on doing a great job. During this time as well she felt her heart beating so fast that it hurt her.
The talk of Love

As time went on Arte started to develop feelings for Leo. Not to mention the scene when Veronica and Arte talked about love and where Arte wanted to know what could be the feeling she has been feeling for her master Leo. After talking she thought of the facts and accepted them.
Leo himself felt some feelings for her. When an offer came to Arte to work as a tutor in Venice for a noble family, Leo felt happy but somewhat sad as Arte would be staying away from her. As her master, he could not stop her for mere reasons and instead told her to be independent as in the future no one would be going to help her feed. As Arte took the offer and went to Venice. There while she stays in her room she often misses Leo and her paintings.
Did they end up together?

After when her contract of working for a half year she was very excited to go back to the city of Florence and Leo. When she reached the atelier she heard Leo's unwellness and went to search for him. She then gets to know that he was working on a ceiling of a church for the Easter festival.
Arte took the task and many others came out to help her. After the work was done Leo came to the church and saw the painting and he was quite proud to see the work by Arte and the Venice style was used in many parts of the painting and complimented on it.
Leo said to her that he thought she would not be coming back and asked for her reason. Arte replied and said she wanted to paint like Leo and wanted to be his apprentice for the time being. Leo smiled at her and said that now she is in his care and vice versa.
They both looked at each other and smiled with loving eyes. Even though they did not get into any relationship other than master and apprentice, we know that both had feelings for each other.
Thanks for reading!!